Integrated circuit packaging

ABSTRACT

An integrated circuit package and methods for packaging an integrated circuit. In one example, a method for packaging an integrated circuit includes connecting input/output pads of a first die to terminals of a lead frame via palladium coated copper wires. An oxygen plasma is applied to the first die and the palladium coated copper wires. The first die and the palladium coated copper wires are encapsulated in a mold compound after application of the oxygen plasma.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 62/611,818, filed Dec. 29, 2017, entitled “IntegratedCircuit Packaging,” which is hereby incorporated herein by reference inits entirety.

BACKGROUND

Isolation is used in electrical circuits to prevent the flow of directcurrents and undesirable alternating currents between two parts of asystem. While preventing flow of undesirable currents between parts of asystem, isolation may allow signal transfer between the isolated partsof the system. Isolation may be provided by integrated circuits referredto as isolators. Isolators may be included in electronic systems forsafety reasons and/or to protect electronic components of the systems.For example, where two systems need to communicate, but the systems havegrounds that may be at different potentials, communication may bethrough an isolator that is tied to the grounds of both systems butallows no current flow between the grounds. Various types of isolatorsmay include optical coupling, capacitive coupling, inductive coupling,or other types of coupling to isolate systems while allowingcommunication between the systems.

Like other integrated circuits, isolators are provided in packages thattypically include at least one die and at least one substrate or leadframe to which the die is affixed by an attachment medium such as solderor epoxy. The substrate facilitates electrical attachment of the die toother circuits. The various components of the integrated circuit packageare generally encased in a protective mold compound, such as epoxy.

SUMMARY

An integrated circuit package and methods for packaging an integratedcircuit are disclosed herein. In one example, a method for packaging anintegrated circuit includes connecting input/output pads of a firstintegrated circuit die to terminals of a lead frame via palladium coatedcopper (PCC) wires. An oxygen plasma is applied to the first integratedcircuit die and the PCC wires. The first integrated circuit die and thePCC wires are encapsulated in a mold compound after application of theoxygen plasma.

In another example, an integrated circuit package includes a first die,a second die, a lead frame, palladium coated copper bond wires, goldbond wires, and encapsulation material. The PCC bond wires conductivelycouple the first die and the second die to the lead frame. The gold bondwires conductively couple the first die to the second die. Theencapsulation material encases the first die, the second die, the leadframe, the PCC bond wires, and the gold bond wires.

In a further example, a method for packaging an integrated circuitincludes connecting input/output pads of a first die to terminals of alead frame via PCC wires. The first die is connected to a second die viagold wires. The lead frame, the first die, the second die, the goldwires, and the PCC wires are encapsulated in a mold compound.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 shows an Energy Dispersive Spectroscopy (EDS) spectrum of apalladium coated copper bond wire prior to application of a plasma inaccordance with various examples;

FIG. 2 shows an EDS spectrum of a palladium coated copper bond wireafter application of argon plasma in accordance with various examples;

FIG. 3 shows a flow diagram for a method for packaging an integratedcircuit in accordance with various examples;

FIG. 4 shows an EDS spectrum of a palladium coated copper bond wireafter application of oxygen plasma in accordance with various examples;and

FIG. 5 shows a diagram of an isolator circuit assembly connected withpalladium coated copper bond wires in accordance with various examples.

DETAILED DESCRIPTION

Certain terms have been used throughout this description and claims torefer to particular system components. As one skilled in the art willappreciate, different parties may refer to a component by differentnames. This document does not intend to distinguish between componentsthat differ in name but not function. In this disclosure and claims, theterms “including” and “comprising” are used in an open-ended fashion,and thus should be interpreted to mean “including, but not limited to .. . .” Also, the term “couple” or “couples” is intended to mean eitheran indirect or direct wired or wireless connection. Thus, if a firstdevice couples to a second device, that connection may be through adirect connection or through an indirect connection via other devicesand connections. The recitation “based on” is intended to mean “based atleast in part on.”Therefore, if X is based on Y, X may be a function ofY and any number of other factors. The term “approximately” is intendedto mean within 10% of a stated amount. Thus, approximately 100 specifiesa range of 90-110.

Cost reduction in packaged integrated circuits, such as packagedisolator integrated circuits, is generally desirable. As the cost ofgold bonding wire has risen over time, bonding wire made of copper orother metals has become available as a lower cost alternative to gold.Copper has a number of advantages over gold, but use of copper bond wirepresents a number of challenges. For example, copper oxidizes atrelatively low temperatures. Copper bond wire may be coated withpalladium to inhibit oxidation. Retention of the palladium coatingthroughout the packaging process is important to maintain the integrityof the bond wire.

At various points in the integrated circuit packaging process, theassembled components may be cleaned to prepare the assembly for furtherprocessing. For example, the integrated circuit assembly may be cleanedto facilitate adhesion in an upcoming packaging operation. In someintegrated circuit packaging processes, a circuit assembly may becleaned by application of a plasma. Plasma is an ionized gas that caneffectively remove contaminants from the surfaces of a circuit assembly.For example, plasma may be applied to a die to prepare the die for wirebonding. Plasma is formed by radiating radio frequency power, e.g.,generating a radio frequency oscillating electric field using capacitiveplates or magnetic induction, into a gas.

In packaging of integrated circuits (e.g., isolator integratedcircuits), an argon plasma may be applied to the circuit assembly afterthe die is wire bonded to the package lead frame. The argon plasmacleans the surfaces of the circuit assembly in preparation forencapsulation. If the surfaces of the circuit assembly are notadequately cleaned prior to encapsulation, then friction with thesurfaces of the circuit assembly may cause voids in the encapsulationmaterial to form about the circuit assembly that can lead to failure ofthe packaged integrated circuit. For example, a void may form in an areawith high electric field, with the void residing between high and lowvoltage nodes within the circuit assembly. The void, which can berepresented as a region of poor breakdown strength isolation within thepackaged device between high and low voltage nodes, can reduce thelifetime of the packaged device. Thus, voids in the encapsulationmaterial encasing an isolator should be prevented to reduce prematurefailures in the isolator.

While use of argon plasma presents no issues with gold bond wires, argonplasma can cause palladium and copper to sputter off of palladium coatedcopper (PCC) bond wire. FIG. 1 shows an energy dispersive spectroscopy(EDS) spectrum of a PCC bond wire prior to application of a plasma. Thepeak 102 represents copper signal intensity and the peak 104 representspalladium signal intensity. The peak 102 is lower than the peak 104which indicates that the copper signal intensity is lower than thepalladium signal intensity in the PCC bond wire prior to application ofplasma.

FIG. 2 shows a spectrum of the palladium coated copper bond wire afterapplication of argon plasma. The peak 202 represents copper signalintensity and the peak 204 represents palladium signal intensity. Thespectrum indicates that relative amplitude of the copper and palladiumsignal peaks has changed (as compared to the peaks of FIG. 1). Forexample, the peak 202 may not be lower than the peak 204. The change inthe relative amplitude of the copper and palladium signal peaks iscaused by the sputtering of palladium from the PCC bond wire by theargon plasma.

Embodiments of the present disclosure avoid or reduce the loss ofpalladium from PCC bond wires without increasing the formation of voidsin the encapsulation material. The packaging methods disclosed hereininclude a cleaning process that employs an oxygen plasma, rather than anargon plasma, to clean the circuit assembly after the palladium coatedcopper bond wires are attached. The oxygen plasma cleans the circuitassembly, and produces a low wetting angle on the surfaces of thecircuit assembly in preparation for application of encapsulationmaterial. Because of the low wetting angle produced by the oxygenplasma, voids are averted in the contact area of the circuit assemblyand the encapsulation material, which in turn reduces the failure rateof the packaged device. Use of PCC, rather than gold, bond wiressubstantially reduces the cost of the packaged device.

FIG. 3 shows a flow diagram for a method 300 for packaging an integratedcircuit in accordance with various examples. Though depictedsequentially as a matter of convenience, at least some of the actionsshown can be performed in a different order and/or performed inparallel. Additionally, some embodiments may perform only some of theactions shown. In some embodiments, the integrated circuit packagedusing the method 300 is an isolator.

In block 302, a first die is attached to a package lead frame, and asecond circuit die is attached to a package lead frame. In someembodiments one or more circuit dice are attached to a package leadframe in block 302. The one or more die may be attached to the leadframe with an epoxy-based adhesive, solder, or another bonding agent orattachment device. FIG. 5 shows a diagram of an isolator circuit package500. In FIG. 5, a die 504 is attached to a lead frame 502, and a die 506is attached to the lead frame 502.

In block 304, an argon plasma is generated and the argon plasma isapplied to the package produced in block 302. That is, the argon plasmais applied to the lead frame and the one or more dice affixed to thelead frame to clean the surfaces of the lead frame and the one or moredice affixed to the lead frame in preparation for attachment of bondwires.

In block 306, input/output pads of the one or more dice attached to thelead frame are conductively connected to terminals of the lead frameusing PCC bond wire. For example, a first end of a PCC bond wire isattached to an input/output pad of a die and a second end of the PCCbond wire is attached to a terminal of the lead frame. In FIG. 5, afirst end of the PCC bond wire 510 is attached to the input/out pad 508of the die 506 and a second end of the PCC bond wire 510 is connected tothe terminal 512 of the lead frame 502. Some terminals (e.g., powerterminals) of the lead frame 502 may be connected to a die (504 or 506)by multiple PCC bond wires. The PCC bond wire may be attached to theinput/output pads of the one or more dice and to the terminals of thelead frame using welding operations such as ball bonding, stitchbonding, and/or wedge bonding.

In block 308, the first die 504 is conductively connected to the seconddie 506. In some embodiments, gold bond wire is used to conductivelyconnect the first die 504 to the second die 506. In some embodiments,palladium coated copper bond wire is used to conductively connect thefirst die 504 to the second die 506. FIG. 5 shows the die 504 connectedto the die 506 using gold bond wires 514. Attachment of the bond wiresconnecting the first die 504 to the second die 506 may be by welding. Insome embodiments (e.g., if the dice 504 and 506 form an isolator), afirst end of the bond wire 514 is connected to a plate of a capacitorformed on the first die 504 and a second end of the bond wire 514 isconnected to a plate of a capacitor formed on the second die 506.Communication between the first die 504 and the second die 506 occursvia the capacitors embedded within each circuit connected by the bondwire 514. The capacitors provide galvanic isolation between the firstdie 504 and the second die 506.

In block 310, an oxygen plasma is generated and the oxygen plasma isapplied to the package produced in block 308. That is, the oxygen plasmais applied to the lead frame 502, the one or more dice (e.g., die 504and die 506) affixed to the lead frame 502, the PCC bond wires 510 thatconnect the lead frame 502 to the one or more dice, and the bond wires514 that connect the dice to one another. The oxygen plasma cleans thesurfaces of the package in preparation for encapsulation. Palladium isnot sputtered from the PCC bond wires 510 by the oxygen plasma. Wettingangles achieved on surfaces of the lead frame 502, the one or more dice(e.g., 504 and 506) affixed to the lead frame 502, the PCC bond wires510 that connect the lead frame 502 to the one or more dice, and thebond wires 514 that connect the dice to one another after application ofthe oxygen plasma are comparable to those obtained using argon plasma(e.g., less the)30°.

FIG. 4 shows a spectrum of a PCC bond wire after application of oxygenplasma in accordance with various examples. The peak 402 representscopper signal intensity and the peak 404 represents palladium signalintensity. Like the spectrum for the pre-plasma palladium coated copperbond wire shown in FIG. 1, the peak 402 is lower than the peak 404 whichindicates that the copper signal intensity is lower than the palladiumsignal intensity. Thus, application of the oxygen plasma does notsputter palladium from the PCC bond wire as does the argon plasma, whichresults in a uniform amount of palladium coating the copper wires.

In block 312, after application of the oxygen plasma, an encapsulationmaterial (e.g., an epoxy mold compound) is molded around the lead frame502, the one or more dice (e.g., die 504 and die 506) affixed to thelead frame 502, the PCC bond wires 510 that conductively connect thedice to the lead frame, and the bond wires 514 that connect the dice toone another. The encapsulation material encases or surrounds the leadframe, dice, bonding wires, etc., while electrical contacts (e.g. pins)of the lead frame remain exposed for use in connecting the packagedintegrated circuit to other systems. Because of the low wetting anglesproduced by the oxygen plasma, voids at the interface of theencapsulation material and the circuit assembly are reduced.

An example of the parameters applied to generate the argon plasma inblock 304 and the oxygen plasma in block 310 is shown in Table 1 below.

TABLE 1 Argon Plasma Oxygen Plasma RF Power 450 watts 250 watts Pressure740 mTor 200 mTor Time  60 s  60 s Argon  92 s.c.c.m.  0 s.c.c.m. Oxygen230 s.c.c.m.  70 s.c.c.m.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

What is claimed is:
 1. A method for packaging an integrated circuit, comprising: connecting input/output pads of a first die to terminals of a lead frame via palladium coated copper wires; applying an oxygen plasma to the first die and the palladium coated copper wires; encapsulating the first die and the palladium coated copper wires in a mold compound after application of the oxygen plasma.
 2. The method of claim 1, further comprising applying an argon plasma to the first die prior to the connecting.
 3. The method of claim 1, further comprising connecting the first die to a second die via gold bond wires prior to applying the oxygen plasma.
 4. The method of claim 3, further comprising connecting a capacitor of the first die to a capacitor of the second die via one of the gold bond wires.
 5. The method of claim 1, further comprising connecting the first die to a second die via palladium coated copper wires prior to applying the oxygen plasma.
 6. The method of claim 1, further comprising connecting input/output pads of a second die to terminals of the lead frame via palladium coated copper wires.
 7. The method of claim 6, wherein applying the oxygen plasma comprises applying the oxygen plasma to the second die and the palladium coated copper wires connecting the input/output pads of the second die to terminals of the lead frame.
 8. The method of claim 6, wherein the encapsulating comprises encapsulating the second die and the palladium coated copper wires connecting the input/output pads of the second die to terminals of the lead frame in the mold compound.
 9. The method of claim 1, further comprising radiating approximately 250 watts of radio frequency power into oxygen gas to produce the oxygen plasma.
 10. An integrated circuit, comprising: a first die; a second die; a lead frame; palladium coated copper bond wires that conductively couple the first die and the second die to the lead frame; gold bond wires that conductively couple the first die to the second die; and encapsulation material surrounding the first die, the second die, the lead frame, the palladium coated copper bond wires, and the gold bond wires.
 11. The integrated circuit of claim 10, wherein the first die comprises a capacitor connected to one of the gold bond wires.
 12. The integrated circuit of claim 11, wherein the second die comprises a capacitor connected to the one of the gold bond wires.
 13. The integrated circuit of claim 10, wherein an amount of palladium coating the palladium coated copper wires disposed within the encapsulation material is uniform over a surface of the palladium coated copper wires.
 14. A method for packaging an integrated circuit, comprising: connecting input/output pads of a first die to terminals of a lead frame via palladium coated copper wires; connecting the first die to a second die via gold wires; and encapsulating the lead frame, the first die, the second die, the gold wires, and the palladium coated copper wires in a mold compound.
 15. The method of claim 14, further comprising applying an oxygen plasma to the first die, the second die, and the palladium coated copper wires after connecting the input/output pads to the terminals of the lead frame via the palladium coated copper wires.
 16. The method of claim 15, wherein applying the oxygen plasma comprises applying the oxygen plasma to the gold wires connecting the first die to the second die.
 17. The method of claim 14, further comprising connecting input/output pads of the second die to terminals of the lead frame via palladium coated copper wires.
 18. The method of claim 17, wherein applying the oxygen plasma comprises applying the oxygen plasma to the palladium coated copper wires connecting input/output pads of the second die to terminals of the lead frame.
 19. The method of claim 14, further comprising applying an argon plasma to the first die and the second die prior to connecting the input/output pads to the terminals of the lead frame via the palladium coated copper wires.
 20. The method of claim 14, further comprising radiating approximately 250 watts of radio frequency power into oxygen gas to produce the oxygen plasma. 